Self-Standing Riser System Having Multiple Buoyancy Chambers

ABSTRACT

A multi-tiered self-standing riser system includes one or more intermediate buoyancy chambers configured to provide an upward lifting force on strings of associated riser assemblies. The intermediate chambers have either an open-bottomed or closed container design. The chambers can further include an auxiliary buoyant material designed to either mix with or contain pressurized fluids injected into the chambers. The self-standing riser system further includes a lower riser assembly affixed to a primary well-drilling fixture. The system also includes an upper riser assembly and one or more additional buoyancy chambers disposed in either direct or indirect communication with one another, as well as with drilling, production and exploration equipment as required by associated operations.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Non-Provisionalapplication Ser. No. 12/274,124 filed Nov. 19, 2008, still pending,which claims the benefit of prior U.S. Provisional Application No.61/003,647, filed Nov. 19, 2007.

FIELD OF THE INVENTION

The present invention relates generally to self-standing riserassemblies utilized during oil and gas exploration and productionoperations, and in a particular though non-limiting embodiment, to aself-standing riser system equipped with multiple buoyancy chamberssuitable for deployment in a variety of water depths and sea conditions.

BACKGROUND OF THE INVENTION

Self-standing risers (hereinafter “SSR”) are employed in the oil and gasindustry to suspend production and injection lines from subseaproduction units, and to support holding tendons associated withfloating offshore structures. Known SSR can be used to facilitatestandard “shallow-water” (e.g., between 0 feet and around 600 feet ofwater) drilling units and cost effective production facilities byplacing blow-out preventers and production trees on top of a buoyancychamber.

The conventional approach to the SSR design has been to employ one largebuoyancy chamber that supports the riser or tendon loads. However, thisapproach has led to increased costs associated with the construction andinstallation of the buoyancy chambers. Such factors have resulted in alack of significant SSR system development by operators who couldrealize a broad spectrum of associated benefits. Nonetheless, theindustry as a whole desires a reduction in oil and gas production costs,a decrease in time delays for drilling exploration wells, and increaseddevelopment of previously discovered fields. There is, therefore, along-felt but unmet need for smaller, more flexible riser systemscapable of more rapid manufacture and deployment that assist in theprofitable development of previously under produced oil and gas fields.

SUMMARY OF THE INVENTION

A self-standing riser system suitable for deepwater oil and gasexploration and production is provided, the system including a lowerriser assembly disposed in communication with a primary well-drillingfixture; one or more intermediate buoyancy chambers disposed incommunication with the lower riser assembly and one or more portions ofintermediate riser assembly, wherein one or more of the buoyancychambers further includes an open-bottomed lower surface portion; and anupper riser assembly disposed in communication with one or more upperbuoyancy chambers, wherein one or more of the upper buoyancy chambersfurther includes a fully enclosed portion.

Ballast loads for the chambers; stress joints for the riser assemblies;methods and means of system deployment and maintenance; access toblow-out preventers, wellheads and production trees; and various systeminterconnections are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments disclosed herein will be better understood, and numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1A is a schematic diagram of a self-standing riser system equippedwith an open-bottom buoyancy chamber in calm waters, according to anexample embodiment known in the prior art.

FIG. 1B is a schematic diagram of a self-standing riser system equippedwith an open-bottomed buoyancy chamber that is nearing its spill point.

FIG. 1C is a schematic diagram of a self-standing riser equipped with anopen-bottomed buoyancy chamber that has tilted beyond its spill point.

FIG. 2 is a schematic diagram depicting the effects of pressure,temperature and depth on a closed-bottom buoyancy chamber.

FIG. 3 is a schematic diagram of a self-standing riser system comprisingmultiple buoyancy chambers, according to example embodiments of thepresent invention.

FIG. 4 is a schematic diagram depicting the installation of aself-standing riser system comprising multiple buoyancy chambers,according to example embodiments of the invention.

DETAILED DESCRIPTION

There are presently two known types of submersible buoyancy chamberssuitable for oil and gas exploration and production: a closed containerdesign, and an open-bottomed design. Both types of chambers, ifpressurized and secured by a riser, will exert an upward lifting forceon the riser. Certain embodiments also comprise features lendingadjustability to the system, as may be known to those of skill in theart.

The closed container design is similar in some respects to a submarine,in that there are typically one or more ballast chambers used to house afluid, such as a light gas, seawater, etc. Once a desired ratio offluids is achieved, the chamber is closed off by valves or other meansknown in the art.

An open-bottomed buoyancy chamber includes many design functions similarto those of the closed container design. However, once desired buoyancycharacteristics are achieved, fluid disposed within the chamber issimply trapped by the sides and top thereof.

FIG. 1A illustrates a known, open-bottomed, buoyancy chamber disposed incommunication with an SSR and filled with a fluid, for example, apressurized gas. As seen, a combination of calm water currents, minimalexternal forces, and a sufficient amount of buoyancy applied to the SSRresults in minimal lateral displacement force. Accordingly, the buoyancychamber illustrated in FIG. 1A experiences little or no tilt relative toits vertical axis, and fluid contained within the chamber remainsenclosed.

If, however, a sufficiently large enough force is applied to thechamber, such as a strong current as depicted in FIG. 1B, the SSR willbegin to tilt away from its vertical axis. FIG. 1B also illustrates howthe fluid contained within the chamber has shifted relative to thesystem's tilt away from its vertical axis. However, the chamber canaccommodate a tilt of up to a certain critical angle (which dependslargely on its design dimensions) before the critical spill point angleis reached, and fluid begins to escape from the chamber.

FIG. 1C further illustrates how the spill rate of the gas containedwithin an open-bottomed buoyancy chamber will increase as the criticaltilt angle is reached and exceeded. In particular, spillage will resultin even greater loss of buoyancy, and therefore a proportionatelyincreasing tilt angle, which will cause more and more gas to escape fromthe chamber. Eventually, enough gas escapes that the buoyant force isreduced to the point where the chamber can no longer support the riser,thereby causing the system to fail.

Despite such drawbacks, open-bottomed chambers can operate at extremewater depths with a reduced concern of structural collapse than a closedsystem, since the open design allows fluid pressures within the chamberto equalize with surrounding pressures at even great depths.Furthermore, the open-bottomed design has less overall system weight dueto a reduction in required construction materials, since there is nobottom, and the remainder of the shell will require less thickness andreinforcement in order to withstand deepwater fluid pressures.

In contrast, closed container buoyancy chambers do not suffer as greatlyfrom the problem of tilting caused by currents and surface effects, andare typically the appropriate design choice in areas where currents andsurface effects are significant enough to cause major lateraldisplacement from the vertical axis. However, if either of the describedbuoyancy chambers sustain a leak (for example, a leak caused bycontainer breach, valve malfunction, etc.), the gas or other fluid willescape and the SSR can fail, as illustrated in FIG. 1C.

Closed container buoyancy chambers must also be robust enough to offsetexternal forces such as deepwater fluid pressure. As illustrated in FIG.2, such chambers must, as a threshold matter, have sufficient structuralintegrity and wall thickness to resist expected pressures that mightcause a collapse of the chamber's outer shell. Moreover, when deployinga closed buoyancy chamber filled with a gas, the internal gas pressuresand temperatures should be sufficiently proportional to the externalwater pressures and temperatures that an associated pressure ortemperature gradient will not induce an effective change in gas volumewithin the chamber which could cause the chamber's outer shell to crackor collapse.

Typically, SSR systems are constrained to include the use of only asingle buoyancy chamber due to the chamber's large size. However, thelarger buoyancy chamber designs increase the time and cost associatedwith building and deploying the operating system. Moreover, deploymentof large, pressurized chamber at great depths (e.g., >500 ft. or so) canprove to be an exceedingly difficult task. Furthermore, as the diameterof the buoyancy chamber is increased, the probability of structuralfailure and warping caused by handling during construction anddeployment is also increased.

The detailed description that follows includes exemplary systems,methods, and techniques that embody techniques of the presentlyinventive subject matter. However, it will be understood by those ofskill in the art that the described embodiments may be practiced withoutone or more of the specific details disclosed herein. In otherinstances, well-known manufacturing equipment, protocols, structures andtechniques have not been shown in detail in order to avoid obfuscationin the description.

Referring now to the example embodiment depicted in FIG. 3, an SSRsystem 14 is depicted comprising a plurality of subordinate buoyancychambers configured to admit to installation in deeper water depths thanany previously known SSR systems. According to an alternativeembodiment, SSR 14 can be stacked with multiple buoyancy chambers asillustrated in FIGS. 4A, 4B, 4C and 4D. Although illustrated in FIG. 3as a combination of lower SSR assembly 10 and upper SSR assembly 12,embodiments of the overall SSR system 14 can comprise any number ofindividual SSR assemblies.

In the embodiment depicted in FIG. 3, lower SSR assembly 10 is firstdeployed. In one example, a specially designed vessel equippedspecifically to deploy buoyancy chambers and SSR assemblies is used.Following deployment, lower SSR assembly 10 is joined in mechanicalcommunication with a casing wellhead established near the mud-line. In atypical embodiment, the casing wellhead has been preset into a well holebored into an associated seafloor surface.

In further embodiments, one or more intermediate buoyancy chambers 16 isattached to lower SSR assembly 10, thereby providing increased stabilityin deep or turbulent waters. Depending on operating conditions,intermediate buoyancy chamber 16 can comprise a closed-container design,but in most instances will comprise the open-bottomed design for thereasons described above, with the only firm requirement being thatintermediate chamber 16 must in any event be capable of providing thesupport required to control lower SSR assembly 10 and upper SSR assembly14.

In further example embodiments, intermediate buoyancy chamber 16 isdisposed in mechanical communication with either previously known orcustom-designed drilling, production and exploration equipment. Thus,for example, the top and bottom portions of an intermediate buoyancychamber may comprise one or more of a blowout preventer, a productiontree, or a wellhead that functions in a manner similar to the casingwellhead placed near mud-line of the ocean floor. Attachment of thedrilling, production and exploration equipment can be achieved usingeither known or custom connection and fastening members, e.g., hydrauliccouplers, various nut and bolt assemblies, welded joints, pressurefittings (either with or without gaskets), swaging, etc., withoutdeparting from the scope of the invention.

In further embodiments, an upper SSR assembly 12 is deployed anddisposed in mechanical communication with a wellhead, blowout preventer,or production tree (or another, custom-designed device combiningelements of one or more of such devices) installed atop an upper surfaceof the intermediate chamber 16 or a connecting member associatedtherewith. According to other example embodiments, the installationprocess continues until the desired number of such assemblies areinstalled in serial communication with one another in order to achieve astable and efficient SSR system 14, as depicted in FIGS. 4A-4D.

In order to further stabilize the SSR system 14, example embodiments canutilize stress joints 22, as depicted in FIG. 3. Stress joints 22 cancomprise any known material, for example, a plastic, rubber, or metalmaterial, but should in any event be capable of maintaining the SSR 14system's structural integrity and overall stability.

Consistent with the example SSR system 14 illustrated in FIG. 3, aplurality of upper buoyancy chambers 18, 20 includes an open-bottomedchamber 18 and a closed-container type chamber 20. In a one exampleembodiment, at least one of said upper chambers—generally thetopmost—will comprise a closed design, while others in the system,including intermediate chamber 16, will comprise an open-bottomeddesign. In another example embodiment, all of the chambers in the systemare either open or closed, and in still further embodiments,combinations of open and closed chambers are employed across the system.

In some embodiments, the multiple open-bottomed design buoyancy chambersare utilized to facilitate deployment in deeper waters in whichsurrounding fluid pressures are greatest. Other embodiments utilize aplurality of closed-container type chambers disposed near the top of theSSR system 14, thereby improving the system's overall stability andbalance. Such configurations can also help avoid the system's tendencyto tilt away from its vertical axis as a result of external lateralforces, such as a forceful cross-current.

In still further embodiments, a plurality of buoyancy chambers disposedin mechanical communication with upper SSR assembly 12 allows for theoverall SSR system 14 to maintain required functionality and stabilityin varying water depths and conditions, thereby improving its efficiencyand operability.

Further example embodiments comprise a plurality of upper buoyancychambers disposed in mechanical communication with commonly knowndrilling, production and exploration equipment. Thus, for example, thetop and bottom portions of an upper buoyancy chamber may comprise one ormore of a blowout preventer, a production tree, or a wellhead designedto function in a manner similar to the casing wellhead placed nearmud-line of the ocean floor.

In further embodiments, the buoyancy chambers utilized throughout thesystem further comprise auxiliary buoyancy materials, such as syntacticfoam or air filled glass micro-spheres that lend buoyancy to the system.Injecting one or more of these materials within an open-bottomed chamberwill assist in prevention of buoyancy fluid (e.g., gas, liquid, etc.)loss should tilting occur, or if there is a breach or failure of tubing,valves, or other equipment utilized in connection with the buoyancychamber.

In the example embodiment illustrated in FIG. 4A, a deployment vesseldeploys a lower SSR assembly 40 to the ocean floor where it ismechanically disposed in communication with a casing wellhead near themud-line. FIG. 4A further depicts an intermediate buoyancy chamber 41installed atop the SSR assembly 40. Various embodiments of theintermediate buoyancy chamber 41 further comprise one or more previouslyknown or custom-fit attachment mechanisms, such as a combined blowoutpreventer and production tree, so that the intermediate chamber 41 isuseful during operations for purposes other than mere connection with anupper SSR assembly 42. In various other embodiments, a plurality ofintermediate buoyancy chambers 41 are deployed and mechanically disposedin communication with a previously installed SSR assembly or anotherintermediate buoyancy chamber (see, for example, FIGS. 4B-4D).

In FIG. 4C, intermediate SSR assemblies 42 and 44 are deployed anddisposed in mechanical communication with a well-head affixed atopintermediate buoyancy chamber 41. In some example embodiments,additional intermediate buoyancy chambers 41, 43, 45 serve as additionalsupport and connection components for the intermediate SSR assemblies.Such redundant embodiments can achieve heretofore unknown SSR systemdepths of more than 15,000 ft. with the addition of multipleintermediate SSR assemblies.

In the example embodiment depicted in FIG. 4D, a final SSR assembly 46is deployed to complete the SSR system 50. FIG. 4D further depicts anembodiment employing a plurality of buoyancy chambers 47 atop SSRassembly 46 in order to complete the overall SSR system 50. Aspreviously discussed, embodiments of the plurality of buoyancy chambers47 can comprise a mixture of open-bottomed and closed-container designs,or any other configuration made desirable by operating conditions,including of course the installation of only a single such chamber.

The foregoing specification is provided for illustrative purposes only,and is not intended to describe all possible aspects of the presentinvention. Moreover, while the invention has been shown and described indetail with respect to several exemplary embodiments, those of ordinaryskill in the art will appreciate that minor changes to the description,and various other modifications, omissions and additions may also bemade without departing from the spirit or scope thereof.

1. A self-standing riser system suitable for deepwater oil and gasexploration and production, said system comprising: a lower riserassembly disposed in communication with a primary well-drilling fixture;one or more intermediate buoyancy chambers disposed in communicationwith said lower riser assembly and one or more portions of intermediateriser assembly, wherein one or more of said buoyancy chambers furthercomprises an open-bottomed portion; and an upper riser assembly disposedin communication with one or more upper buoyancy chambers, wherein oneor more of said upper buoyancy chambers further comprises anopen-bottomed portion.
 2. The self-standing riser of claim 1, whereineach of said open-bottomed intermediate buoyancy chambers furthercomprises a fluid ballast.
 3. The self-standing riser system of claim 2,wherein said fluid ballast further comprises a gas ballast.
 4. Theself-standing riser system of claim 2, wherein said fluid ballastfurther comprises a liquid ballast.
 5. The self-standing riser system ofclaim 2, wherein said fluid ballast further comprises a ballastincluding both a liquid and a gas.
 6. The self-standing riser system ofclaim 2, wherein said fluid ballast further comprises an auxiliaryballast that lends additional pressure and density to said fluid.
 7. Theself-standing riser system of claim 6, wherein said auxiliary ballastretards the escape of fluid from within said open-bottomed intermediatebuoyancy chambers in the event said chambers tilt beyond a criticalangle relative to its vertical axis.
 8. The self-standing riser systemof claim 1, wherein one or more of said intermediate buoyancy chambersfurther comprises a closed bottom portion.
 9. The self-standing risersystem of claim 1, wherein one or more of said upper buoyancy chambersfurther comprises a closed bottom portion.
 10. The self standing risersystem of claim 1, wherein each of said open-bottomed upper buoyancychambers further comprises a fluid ballast.
 11. The self-standing risersystem of claim 10, wherein said fluid ballast further comprises a gasballast.
 12. The self-standing riser system of claim 10, wherein saidfluid ballast further comprises a liquid ballast.
 13. The self-standingriser system of claim 10, wherein said fluid ballast further comprises aballast including both a liquid and a gas.
 14. The self-standing risersystem of claim 10, wherein said fluid ballast further comprises anauxiliary ballast that lends additional pressure and density to saidfluid.
 15. The self-standing riser system of claim 14, wherein saidauxiliary ballast retards the escape of fluid from within saidopen-bottomed intermediate buoyancy chambers in the event said chamberstilt beyond a critical angle relative to its vertical axis.
 16. Theself-standing riser system of claim 1, wherein one or more lengths ofsaid lower riser assembly and said upper riser assembly furthercomprises one or more stress joints for absorbing stress accumulatedwithin said lengths of said assemblies.